MCCUNE, A.R.; Cornell University: Insights from natural variation in Danio rerio

Most genetic screens of zebrafish have relied on artificial mutagenesis to generate genetic mutants. However, by inbreeding individuals from wild populations of zebrafish, many natural mutations can be recovered. Study of such natural mutants can be just as informative about genetic and developmental mechanisms as study of artificially generated mutants, but, in addition, study of natural mutants will characterize the actual variation available to the evolutionary process in nature. Study of genetic and phenotypic variation and the mechanistic relations between them is at least as important to understanding the evolutionary process as is the study of natural selection. Comparative (interspecific) study of phenotypes and genotypes reveals evolutionary change that has already occurred; intraspecific study of variation informs us about evolutionary potentials. To the extent that intraspecific variation in living populations parallels interspecific variation, study of intraspecific variation can also suggest genetic/developmental mechanisms that have produced evolutionary change. In a screen of natural mutants in wild-caught zebrafish, we discovered that the phenotype of most recessive lethals exposed by inbreeding included loss or non-inflation of the gas bladder. While this trait has been little studied and the individuals lacking gas bladders have been discarded in other screens, loss of the gas bladder is fascinating from an evolutionary perspective. Among teleosts, the dominant group of living fishes, loss of the gas bladder has occurred many times independently. For the widely convergent loss of gas bladders seen among teleost fishes, both adaptive evolution and developmental bias are implicated. It remains an evolutionary developmental mystery why the loss of the gas bladder can occur so easily and in so many genetically different ways.